1. Field of the Invention
The present invention relates to a method and apparatus for compensating for phase and amplitude distortion in an output signal generated when a complex signal is modulated onto a radio frequency carrier signal using IQ modulation techniques or when a complex signal is demodulated from a radio frequency carrier signal, again using IQ modulation techniques.
2. Discussion of Prior Art
The complex signal is broken up into its real part, the I-signal and an imaginary part, the Q-signal. The I-signal is amplitude modulated onto a local oscillator signal using a first double sideband mixer and the Q-signal is amplitude modulated onto a phase quadrature local oscillator signal (90xc2x0 of phase with the local oscillator signal) using a second double sideband mixer. The outputs of the two double sideband mixers are then combined and the resultant modulated carrier signal carries the complex signal as amplitude and phase variations. The I and Q-signals can be recovered by demodulation in an IQ demodulation arrangement with respect to a local oscillator signal and a phase quadrature local oscillator signal respectively.
When a complex signal is modulated onto a carrier using an IQ modulator, the IQ modulator will behave substantially linearly when the input signal levels are low. As the level of the input signals increase an IQ modulator will become more non-linear and the output modulated carrier signal becomes distorted. Intermodulation distortion becomes a problem because some of the third order intermodulation products will have a frequency which is close to that of the desired signals and so are difficult to remove by filtering.
It is possible to operate an IQ mudulator by applying low enough level input signals such that the output signal can be modelled as the desired signal plus a small amount of third order distortion. Then a compensatory cubic distortion can be applied in order to improve the linearity of the output signal from the IQ modulator.
Cubic predistortion has been used in an IQ modulator by cubicly predistorting the complex signal. This is disclosed in the article entitled xe2x80x9cTransmitter linearisation using composite modulation feedbackxe2x80x9d in Volume 32, Number 23 of Electronics Letters dated Nov. 7, 1996 in which the cubic predistortion is combined with a negative feedback loop. However, this cubic predistortion is of the opposite sign in upper and lower sidebands whereas the IQ modulator phase distortion is of the same sign in the upper and lower sidebands. This means that cancellation of distortion in both sidebands simultaneously is not possible.
The object of the present invention is to provide a distortion compensating arrangement for applications in which a complex signal is modulated onto a single carrier signal or in which a complex signal is recovered from a single carrier signal, using IQ mudulation techniques and which overcomes the problems discussed above.
According to the present invention there is provided distortion compensating apparatus for use in IQ modulation and demodulaton techniques wherein a first distortion arrangement distorts the I-signal by adding to the I-signal a first weighted function of the I-signal and a first weighted function of the Q signal and a second distortion arrangement distorts the Q-signal by adding to the Q-signal a second weighted function of the I-signal and a second weighted function of the Q-signal, such that the weighted functions of the I-signal are independent of the Q-signal and the weighted functions of the Q-signal are independent of the I-signal. In the prior art any distortion for improving linearity has distorted both components of the complex signal identically, whereas according to the present invention the distortion takes place on the I and Q components independently. This makes it possible to compensate for distortion in both sidebands simultaneously.
According to a preferred embodiment of the present invention the distortion compensating apparatus, comprises;
a first operator arrangement for generating a function of the I-signal,
a second operator arrangement for generating a function of the Q-signal, and
signal processing means for performing a weighted summation of the I-signal, the function of the I-signal and the function of the Q-signal to generate a distortion compensated I-signal output, and for performing a weighted summation of the Q-signal, the function of the Q-signal and the function of the I-signal to generate a distortion compensated Q-signal output.
By taking the I and Q signals which are to be amplitude modulated onto a single carrier signal by an IQ mudulator, and predistorting the I-signal by adding an appropriately weighted amount of a function of the I-signal and a function of the Q-signal and also predistorting the Q-signal by adding an appropriately weighted amount of a function of the I-signal and a function of the Q-signal, distortion can be substantially eliminated in both the upper and lower sidebands of the modulated carrier signal output from the IQ modulator.
Preferably the function is a polynomial and preferably the polynomial is cubic. Cubic distortion is the dominant type of distortion for most components and so a compensating cubic polynomial is preferred.
However, particularly in analogue processing, approximations to a cubic distortion, which effectively applies an approximation to a cubic polynomial to the I and Q-signals, may be simpler to implement. In certain cases higher order polynomials may be appropriate to provide distortion compensation.
The apparatus according to the present invention can be used to predistort I and Q-signals which are to be input into an IQ modulator or to post distort I and Q-signals which have been recovered from an IQ demodulator .
It is preferred that least one multiplier is used to multiply the functions of the I and Q-signals by a weighting factor. This provides the weighting ready for the subsequent summation. Then, it is preferred that at least one adder adds selected outputs of the one or more multipliers in order to perform the weighted summations.
In a preferred embodiment a first multiplier multiplies the function of the I-signal with a first weighting factor and a second multiplier multiplies the function of the Q-signal with a second weighting factor and an adder adds the output from the first multiplier and the second multiplier to the I-signal. In a preferred embodiment, a third multiplier multiplies the function of the I-signal with a third weighting factor and a fourth multiplier multiplies the function of the Q-signal with a fourth weighting factor and an adder adds the output from the third multiplier and the fourth multiplier to the Q-signal.
At least one of the multipliers may be implemented as a look up table and at least one of the operator arrangements can be implemented as a look up table in order to efficiently implement the circuitry required to perform the weighted summation.
According to a second aspect of the present invention there is provided a device for adjusting a composite IQ modulator or demodulator, which composite IQ modulator or demodulator comprises a distortion compensating apparatus as described above and an IQ modulator or demodulator.
The adjusting device comprises a signal source for generating test signals, a spectrum analyzer for measuring the level of distortion in the output of the IQ modulator or demodulator under test and a control processor for controlling the weighting factors. In this way the weighting factors can be varied until the distortion level is minimised.
According to a third aspect of the present invention there is provided a method for improving linearity in IQ modulation and demodulation techniques wherein the I-signal is distorted by adding to the I-signal a first weighted function of the I-signal and a first weighted function of the Q-signal and the Q-signal is distorted by adding to the Q-signal a second weighted function of the I-signal and a second weighted function of the Q-signal, such that the function of the I-signal is independent of the Q-signal and the function of the Q-signal is independent of the I-signal.
In a preferred embodiment the method comprises;
generating a function of the I-signal,
generating a function of the Q-signal,
performing a weighted summation of the I-signal, the function of the I-signal and the function of the Q-signal to generate a distortion compensated I-signal output,
and performing a weighted summation of the Q-signal, the function of the Q-signal and the function of the I-signal to generate a distortion compensated Q-signal output.
Preferably the function is a polynomial and preferably the polynomial is cubic. According to a preferred embodiment of the method at least one of the functions of the signals is multiplied by one or more weighting factors. Thereafter selected multiplied functions of the signals can be added to perform the weighted summation.
It will be apparent that the distortion compensation according to the present invention can be carried out by digital or analogue processing of the I and Q-signals or by a combination of digital and analogue processing stages.